A known fuel sensor is configured to detect a property of fuel such as a concentration of ethanol contained in fuel for an internal combustion engine of a vehicle. An electronic control unit (ECU) for an engine controls a quantity of fuel injection and a time point of fuel injection according to the property of fuel detected with the fuel sensor. The present configuration enhances drivability of the vehicle and reduces pollution caused by exhaust gas. Patent Document 1 discloses a fuel sensor including an external electrode and a center electrode housed in a housing. The external electrode and the center electrode are immersed in fuel, which flows through the housing, to form a capacitor therebetween. The fuel sensor of Patent Document 1 charges the capacitor with electricity and discharges the charged electricity from the capacitor thereby to detect a capacitance of fuel, which flows between the electrodes. The fuel sensor computes a specific inductive capacity of fuel from the capacitance and detects a concentration of ethanol contained in fuel according to the computed specific inductive capacity.